The invention relates to a method for the position detection of the strip edge of a material web by means of an ultrasonic detector disposed in the strip edge region and consisting of a transmitter and a receiver, the received sound waves being transformed into an electrical signal.
Various types of equipment are known for edge- or center-exact guiding of material webs. In pneumohydraulic control devices (German Patent No. 15 74 638 and German Publication No. 27 30 737), a pneumatic web edge sensor is used which consists essentially of two nozzles arranged one above the other, namely the transmitter nozzle and the receiver nozzle, which are arranged in the edge region of the material web. The receiver pressure varies as a function of the position of the material web and acts on a diaphragm drive, which in turn conducts a liquid stream proportional to the receiver pressure to a setting element, which is formed as a rule, by a control roller or adjustable reel.
Often, however, the hydraulic amplification is not sufficient to be able to adjust the response sensitivity of the control device in accordance with the technical requirements. In these cases, the response sensitivity can be increased only in that the receiver pressure is increased by increase of the transmitter pressure. When the material webs are thin, however, an undesirable blowing away of the material web takes place.
To avoid this disadvantage, it is a known practice also to use, instead of a pneumatic strip edge sensor, an optical strip edge sensor which consists of a photo-electrical system. Because, due to special characteristics of the installation, the web edge sensors are often subject to severe dirt accumulation, a cover of dirt on the optic system leads to a change in intensity of the light rays and hence to an erroneous measurement.
Lastly, it is also known to probe the strip edge of a material by means of an ultrasonic device consisting of a transmitter and a receiver (publication ECOSONIC of the firm Endress & Hauser GmbH & Co., Maulburg). With the known ultrasonic detectors, however, major falsifications of the measured signal occur due to the fact that the receiver receives not only the test beam but also reflected beams not belonging to the immediate beam path. In particular, if the height or level of the strip between the receiver and the transmitter fluctuates to such an extent that a harmonic ratio is reached for the reflected signals, which is in a ratio to the wave length of the radiated sound, the reflected signals are added or subtracted to or from the direct measurement signals, depending on their phase signals, thus leading to a false measurement value.